1. Field of the Invention
This invention relates to hydrogen absorbing alloys, and more particularly to a hydrogen absorbing alloy which can be used to form negative electrodes for use in nickel-metal hydride rechargeable (secondary) batteries.
2. Description of the Related Art
Conventionally, misch metal (hereinafter referred to as xe2x80x9cMmxe2x80x9d) comprising a mixture of rare earth elements such as La, Ce, Pr, Nd and Sm, and nickel-base alloys formed by replacing a part of Ni with various elements are widely used as hydrogen absorbing alloys for forming negative electrodes for use in nickel-metal hydride rechargeable batteries.
It is known that, among others, cobalt-containing alloys are capable of absorbing a relatively large amount of hydrogen, are less liable to particle size reduction in their hydrogen-loaded state, have excellent corrosion resistance in alkalis, and are effective in prolonging the lives of nickel-metal hydride rechargeable batteries when they are used for the negative electrodes thereof.
On the other hand, it is also known that lower cobalt contents are more desirable for an improvement of a high rate discharge property. The reason for this is believed to be that a decrease in cobalt content promotes particle size reduction and hence causes an increase in surface area per unit weight.
In order to solve these problems of the prior art, the present invention provides a hydrogen absorbing alloy which can improve a high rate discharge property while suppressing particle size reduction, exhibits cycle life characteristics equal to or higher than those of conventional alloys even when its cobalt content is decreased, and has a high capacity.
The present invention is based on the discovery that, when a hydrogen absorbing alloy has a relatively high La content and contains an alkaline earth metal (i.e., Mg or Ca) in a relatively small amount above impurity levels, the alloy can improve a high rate discharge property in spite of suppressed particle size reduction while maintaining its high capacity, and can suppress particle size reduction even when its cobalt content is decreased to less than the conventionally known level.
Specifically, the present invention relates to a hydrogen absorbing alloy having a CaCu5 type crystal structure in its principal phase, wherein the La content in the alloy is in the range of 24 to 33% by weight and the Mg or Ca content in the alloy is in the range of 0.1 to 1.0% by weight.
In a preferred embodiment, the present invention also relates to the aforesaid alloy wherein the cobalt content in the alloy is not greater than 9% by weight.
When the hydrogen absorbing alloy of the present invention is used for the negative electrode of an alkaline rechargeable battery, it can increase the capacity of the battery, can improve the high rate discharge property thereof, and can suppress particle size reduction even at low cobalt contents to cause a reduction in battery cost.